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        </div><article class="page single"><h1 class="single-title animated flipInX">Spring源码 - 复杂对象的构建方式 - FactoryBean</h1><div class="post-meta">
            <div class="post-meta-line"><span class="post-author"><a href="https://wlizhi.cc" title="Author" target="_blank" rel="noopener noreffer author" class="author"><i class="fas fa-user-circle fa-fw"></i>Wlizhi</a></span>&nbsp;<span class="post-category">收录于 <a href="/categories/spring/"><i class="far fa-folder fa-fw"></i>spring</a></span></div>
            <div class="post-meta-line"><i class="far fa-calendar-alt fa-fw"></i>&nbsp;<time datetime="2020-11-28">2020-11-28</time>&nbsp;<i class="fas fa-pencil-alt fa-fw"></i>&nbsp;约 3230 字&nbsp;
                <i class="far fa-clock fa-fw"></i>&nbsp;预计阅读 7 分钟&nbsp;<span id="/posts/spring/07-factory-bean/" class="leancloud_visitors" data-flag-title="Spring源码 - 复杂对象的构建方式 - FactoryBean">
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  <ul>
    <li><a href="#1-factorybean-的作用">1 FactoryBean 的作用</a></li>
    <li><a href="#2-factorybean-接口的定义">2 FactoryBean 接口的定义</a></li>
    <li><a href="#3-应用场景">3 应用场景</a></li>
    <li><a href="#4-源码分析">4 源码分析</a>
      <ul>
        <li><a href="#41-初次创建">4.1 初次创建</a></li>
        <li><a href="#42-缓存中获取">4.2 缓存中获取</a></li>
        <li><a href="#43-获取原始bean">4.3 获取原始bean</a></li>
      </ul>
    </li>
  </ul>
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            </div><div class="content" id="content"><p>spring源码系列文章，示例代码的中文注释，均是 copy 自 <a href="https://gitee.com/wlizhi/spring-framework" target="_blank" rel="noopener noreffer">https://gitee.com/wlizhi/spring-framework</a> 。</p>
<p>链接中源码是作者从 github 下载，并以自身理解对核心流程及主要节点做了详细的中文注释。</p>
<hr>
<h2 id="1-factorybean-的作用">1 FactoryBean 的作用</h2>
<p>Spring 中有两种类型的Bean，一种是普通Bean，另一种是工厂Bean 即 FactoryBean。FactoryBean跟普通Bean不同，其返回的对象不是指定类的一个实例，而是该FactoryBean的getObject方法所返回的对象。创建出来的对象是否属于单例由isSingleton中的返回决定。</p>
<p>一般情况下，Spring通过反射机制利用<bean>的class属性指定实现类实例化Bean，在某些情况下，实例化Bean过程比较复杂，如果按照传统的方式，则需要在<bean>中提供大量的配置信息。配置方式的灵活性是受限的，这时采用编码的方式可能会得到一个简单的方案。Spring为此提供了一个org.springframework.bean.factory.FactoryBean的工厂类接口，用户可以通过实现该接口定制实例化Bean的逻辑。FactoryBean接口对于Spring框架来说占用重要的地位，Spring自身就提供了70多个FactoryBean的实现。它们隐藏了实例化一些复杂Bean的细节，给上层应用带来了便利。从Spring3.0开始，FactoryBean开始支持泛型，即接口声明改为FactoryBean<T>的形式</p>
<p>以Bean结尾，表示它是一个Bean，不同于普通Bean的是：它是实现了FactoryBean<T>接口的Bean，根据该Bean的ID从BeanFactory中获取的实际上是FactoryBean的getObject()返回的对象，而不是FactoryBean本身，如果要获取FactoryBean对象，请在id前面加一个&amp;符号来获取。</p>
<h2 id="2-factorybean-接口的定义">2 FactoryBean 接口的定义</h2>
<p>以下是 Spring 中 FactoryBean 接口的定义：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">interface</span> <span class="nc">FactoryBean</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="n">T</span> <span class="nf">getObject</span><span class="o">()</span> <span class="kd">throws</span> <span class="n">Exception</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="n">Class</span><span class="o">&lt;?&gt;</span> <span class="n">getObjectType</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="k">default</span> <span class="kt">boolean</span> <span class="nf">isSingleton</span><span class="o">()</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="kc">true</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><h2 id="3-应用场景">3 应用场景</h2>
<p>FactoryBean 通常是用来创建比较复杂的 bean，一般的 bean 直接用 xml 配置即可，但如果一个 bean 的创建过程中涉及到很多其他的 bean 和复杂的逻辑，用 xml 配置比较困难，这时可以考虑用 FactoryBean。</p>
<p>很多开源项目在集成 Spring 时都使用到 FactoryBean，比如 MyBatis3 提供 mybatis-spring 项目中的 org.mybatis.spring.SqlSessionFactoryBean：</p>
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<pre tabindex="0" class="chroma"><code class="language-xml" data-lang="xml"><span class="line"><span class="cl">    <span class="nt">&lt;bean</span> <span class="na">id=</span><span class="s">&#34;sqlSessionFactory&#34;</span> <span class="na">class=</span><span class="s">&#34;org.mybatis.spring.SqlSessionFactoryBean&#34;</span><span class="nt">&gt;</span>
</span></span><span class="line"><span class="cl">    <span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">&#34;dataSource&#34;</span> <span class="na">ref=</span><span class="s">&#34;dataSource&#34;</span><span class="nt">/&gt;</span>
</span></span><span class="line"><span class="cl">    <span class="c">&lt;!-- 自动扫描mapping.xml文件 --&gt;</span>
</span></span><span class="line"><span class="cl">    <span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">&#34;mapperLocations&#34;</span> <span class="na">value=</span><span class="s">&#34;classpath:mapper/*.xml&#34;</span><span class="nt">&gt;&lt;/property&gt;</span>
</span></span><span class="line"><span class="cl">    <span class="nt">&lt;/bean&gt;</span>
</span></span></code></pre></td></tr></table>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">SqlSessionFactoryBean</span> <span class="kd">implements</span> <span class="n">FactoryBean</span><span class="o">&lt;</span><span class="n">SqlSessionFactory</span><span class="o">&gt;,</span> <span class="n">InitializingBean</span><span class="o">,</span> <span class="n">ApplicationListener</span><span class="o">&lt;</span><span class="n">ApplicationEvent</span><span class="o">&gt;</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl"><span class="c1">// ...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="n">SqlSessionFactory</span> <span class="nf">getObject</span><span class="o">()</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="k">if</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">sqlSessionFactory</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">        <span class="k">this</span><span class="o">.</span><span class="na">afterPropertiesSet</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">    <span class="o">}</span>
</span></span><span class="line"><span class="cl">    <span class="k">return</span> <span class="k">this</span><span class="o">.</span><span class="na">sqlSessionFactory</span><span class="o">;</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="c1">// ...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><h2 id="4-源码分析">4 源码分析</h2>
<h3 id="41-初次创建">4.1 初次创建</h3>
<p>AbstractBeanFactory 通过 getBean 向 IOC 容器获取被管理的 Bean。</p>
<p>AbstractBeanFactory.getBean() -&gt; doGetBean()。</p>
<p>来到 doGetBean()，看 bean 第一次创建实例后，调用的地方：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractBeanFactory</span> <span class="kd">extends</span> <span class="n">FactoryBeanRegistrySupport</span> <span class="kd">implements</span> <span class="n">ConfigurableBeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">final</span> <span class="n">RootBeanDefinition</span> <span class="n">mbd</span> <span class="o">=</span> <span class="n">getMergedLocalBeanDefinition</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">    <span class="k">if</span> <span class="o">(</span><span class="n">mbd</span><span class="o">.</span><span class="na">isSingleton</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">        <span class="n">sharedInstance</span> <span class="o">=</span> <span class="n">getSingleton</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="o">()</span> <span class="o">-&gt;</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="k">try</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">                <span class="c1">//创建Bean实例
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                <span class="k">return</span> <span class="n">createBean</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">mbd</span><span class="o">,</span> <span class="n">args</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">            <span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="n">BeansException</span> <span class="n">ex</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">                <span class="c1">// Explicitly remove instance from singleton cache: It might have been put there
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                <span class="c1">// eagerly by the creation process, to allow for circular reference resolution.
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                <span class="c1">// Also remove any beans that received a temporary reference to the bean.
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                <span class="n">destroySingleton</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">                <span class="k">throw</span> <span class="n">ex</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">            <span class="o">}</span>
</span></span><span class="line"><span class="cl">        <span class="o">});</span>
</span></span><span class="line"><span class="cl">        <span class="cm">/*
</span></span></span><span class="line"><span class="cl"><span class="cm">        TODO 重要程度 5
</span></span></span><span class="line"><span class="cl"><span class="cm">            如果bean不是FactoryBean类型或者beanName以&amp;开头，则直接返回。
</span></span></span><span class="line"><span class="cl"><span class="cm">            否则调用FactoryBean的getObject(),且将返回的bean替换为getObject()的返回值。
</span></span></span><span class="line"><span class="cl"><span class="cm">            FactoryBean接口很重要，具体应用场景见FactoryBean接口注释。
</span></span></span><span class="line"><span class="cl"><span class="cm">         */</span>
</span></span><span class="line"><span class="cl">        <span class="n">bean</span> <span class="o">=</span> <span class="n">getObjectForBeanInstance</span><span class="o">(</span><span class="n">sharedInstance</span><span class="o">,</span> <span class="n">name</span><span class="o">,</span> <span class="n">beanName</span><span class="o">,</span> <span class="n">mbd</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">    <span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>可以看到，在 <code>getSingleton()</code> 之后，又调用了 <code>bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd)</code>，给 bean 重新赋值，这里bean的实例就可能被改变了。</p>
<p>如果当前创建的 bean 是 FactoryBean 类型的，调用 <code>getObjectForBeanInstance()</code> 时，最终会返回 <code>FactoryBean.getObject()</code> 返回的对象，也就是最终 <code>getBean()</code> 返回的是这个 <code>getObject()</code> 获取到的实例。</p>
<p>以下是 <code>getObjectForBeanInstance()</code> 的源码：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractAutowireCapableBeanFactory</span> <span class="kd">extends</span> <span class="n">AbstractBeanFactory</span> <span class="kd">implements</span> <span class="n">AutowireCapableBeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="n">Object</span> <span class="nf">getObjectForBeanInstance</span><span class="o">(</span><span class="n">Object</span> <span class="n">beanInstance</span><span class="o">,</span> <span class="n">String</span> <span class="n">name</span><span class="o">,</span> <span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="nd">@Nullable</span> <span class="n">RootBeanDefinition</span> <span class="n">mbd</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">        <span class="n">String</span> <span class="n">currentlyCreatedBean</span> <span class="o">=</span> <span class="k">this</span><span class="o">.</span><span class="na">currentlyCreatedBean</span><span class="o">.</span><span class="na">get</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">        <span class="k">if</span> <span class="o">(</span><span class="n">currentlyCreatedBean</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="n">registerDependentBean</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">currentlyCreatedBean</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// 从父类方法中获取
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="k">return</span> <span class="kd">super</span><span class="o">.</span><span class="na">getObjectForBeanInstance</span><span class="o">(</span><span class="n">beanInstance</span><span class="o">,</span> <span class="n">name</span><span class="o">,</span> <span class="n">beanName</span><span class="o">,</span> <span class="n">mbd</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">    <span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>这里调用到了父类的方法，继续看父类的方法：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractBeanFactory</span> <span class="kd">extends</span> <span class="n">FactoryBeanRegistrySupport</span> <span class="kd">implements</span> <span class="n">ConfigurableBeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">protected</span> <span class="n">Object</span> <span class="nf">getObjectForBeanInstance</span><span class="o">(</span>
</span></span><span class="line"><span class="cl">			<span class="n">Object</span> <span class="n">beanInstance</span><span class="o">,</span> <span class="n">String</span> <span class="n">name</span><span class="o">,</span> <span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="nd">@Nullable</span> <span class="n">RootBeanDefinition</span> <span class="n">mbd</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// ...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 如果bean不是FactoryBean类型或者beanName以&amp;开头，则直接返回。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="k">if</span> <span class="o">(!(</span><span class="n">beanInstance</span> <span class="k">instanceof</span> <span class="n">FactoryBean</span><span class="o">)</span> <span class="o">||</span> <span class="n">BeanFactoryUtils</span><span class="o">.</span><span class="na">isFactoryDereference</span><span class="o">(</span><span class="n">name</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">return</span> <span class="n">beanInstance</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="n">Object</span> <span class="n">object</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 如果BeanDefinition为null，从缓存中获取，doGetBean() 中传入的就是null
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="c1">// createBean后进入这里的话，传入的就不是null
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="k">if</span> <span class="o">(</span><span class="n">mbd</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="n">object</span> <span class="o">=</span> <span class="n">getCachedObjectForFactoryBean</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="n">object</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="n">FactoryBean</span><span class="o">&lt;?&gt;</span> <span class="n">factory</span> <span class="o">=</span> <span class="o">(</span><span class="n">FactoryBean</span><span class="o">&lt;?&gt;)</span> <span class="n">beanInstance</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">			<span class="k">if</span> <span class="o">(</span><span class="n">mbd</span> <span class="o">==</span> <span class="kc">null</span> <span class="o">&amp;&amp;</span> <span class="n">containsBeanDefinition</span><span class="o">(</span><span class="n">beanName</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="n">mbd</span> <span class="o">=</span> <span class="n">getMergedLocalBeanDefinition</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="kt">boolean</span> <span class="n">synthetic</span> <span class="o">=</span> <span class="o">(</span><span class="n">mbd</span> <span class="o">!=</span> <span class="kc">null</span> <span class="o">&amp;&amp;</span> <span class="n">mbd</span><span class="o">.</span><span class="na">isSynthetic</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 调用FactoryBean的getObject方法
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="n">object</span> <span class="o">=</span> <span class="n">getObjectFromFactoryBean</span><span class="o">(</span><span class="n">factory</span><span class="o">,</span> <span class="n">beanName</span><span class="o">,</span> <span class="o">!</span><span class="n">synthetic</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">object</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>上面代码的尾部，调用到了 <code>object = getObjectFromFactoryBean(factory, beanName, !synthetic)</code>，这个方法中调用了 getObject()。</p>
<p>源码如下（关键点 - 注意源码中高亮部分）：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">FactoryBeanRegistrySupport</span> <span class="kd">extends</span> <span class="n">DefaultSingletonBeanRegistry</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="n">Object</span> <span class="nf">getObjectFromFactoryBean</span><span class="o">(</span><span class="n">FactoryBean</span><span class="o">&lt;?&gt;</span> <span class="n">factory</span><span class="o">,</span> <span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="kt">boolean</span> <span class="n">shouldPostProcess</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="n">factory</span><span class="o">.</span><span class="na">isSingleton</span><span class="o">()</span> <span class="o">&amp;&amp;</span> <span class="n">containsSingleton</span><span class="o">(</span><span class="n">beanName</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="kd">synchronized</span> <span class="o">(</span><span class="n">getSingletonMutex</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="n">Object</span> <span class="n">object</span> <span class="o">=</span> <span class="k">this</span><span class="o">.</span><span class="na">factoryBeanObjectCache</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">				<span class="k">if</span> <span class="o">(</span><span class="n">object</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 调用 factory.getObject();
</span></span></span><span class="line hl"><span class="cl"><span class="c1"></span>					<span class="n">object</span> <span class="o">=</span> <span class="n">doGetObjectFromFactoryBean</span><span class="o">(</span><span class="n">factory</span><span class="o">,</span> <span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="n">Object</span> <span class="n">alreadyThere</span> <span class="o">=</span> <span class="k">this</span><span class="o">.</span><span class="na">factoryBeanObjectCache</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="k">if</span> <span class="o">(</span><span class="n">alreadyThere</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="n">object</span> <span class="o">=</span> <span class="n">alreadyThere</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="k">else</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="k">if</span> <span class="o">(</span><span class="n">shouldPostProcess</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">							<span class="k">if</span> <span class="o">(</span><span class="n">isSingletonCurrentlyInCreation</span><span class="o">(</span><span class="n">beanName</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="c1">// Temporarily return non-post-processed object, not storing it yet..
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>								<span class="k">return</span> <span class="n">object</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span>
</span></span><span class="line"><span class="cl">							<span class="n">beforeSingletonCreation</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">							<span class="k">try</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="c1">// 这里调用了BeanPostProcessor的后置处理
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>								<span class="n">object</span> <span class="o">=</span> <span class="n">postProcessObjectFromFactoryBean</span><span class="o">(</span><span class="n">object</span><span class="o">,</span> <span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span>
</span></span><span class="line"><span class="cl">							<span class="k">catch</span> <span class="o">(</span><span class="n">Throwable</span> <span class="n">ex</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="k">throw</span> <span class="k">new</span> <span class="n">BeanCreationException</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span>
</span></span><span class="line"><span class="cl">										<span class="s">&#34;Post-processing of FactoryBean&#39;s singleton object failed&#34;</span><span class="o">,</span> <span class="n">ex</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span>
</span></span><span class="line"><span class="cl">							<span class="k">finally</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="n">afterSingletonCreation</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span>
</span></span><span class="line"><span class="cl">						<span class="o">}</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// 加入到缓存，缓存位置：BeanFactory的 factoryBeanObjectCache 成员中
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="k">if</span> <span class="o">(</span><span class="n">containsSingleton</span><span class="o">(</span><span class="n">beanName</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line hl"><span class="cl">							<span class="k">this</span><span class="o">.</span><span class="na">factoryBeanObjectCache</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">object</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">						<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="k">return</span> <span class="n">object</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// ...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div></p>
<p>doGetObjectFromFactoryBean() 中，调用到了 getObject()，进入 doGetObjectFromFactoryBean()。
源码中关键代码是这么写的：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl">    <span class="kd">private</span> <span class="n">Object</span> <span class="nf">doGetObjectFromFactoryBean</span><span class="o">(</span><span class="kd">final</span> <span class="n">FactoryBean</span><span class="o">&lt;?&gt;</span> <span class="n">factory</span><span class="o">,</span> <span class="kd">final</span> <span class="n">String</span> <span class="n">beanName</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">BeanCreationException</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// ...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="c1">// 调用FactoryBean的 getObject()，这个方法是实现FactoryBean的类中定义的。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="n">object</span> <span class="o">=</span> <span class="n">factory</span><span class="o">.</span><span class="na">getObject</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// ...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	    <span class="k">return</span> <span class="n">object</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div></p>
<p>由此可以看出，在实例化流程 doGetBean() -&gt; getSingleton() -&gt; getObjectForBeanInstance() 中，创建完 bean 实例后，返回的并不一定是 bean 实例本身，如果这个 bean 实现了 FactoryBean 接口，那么最终会返回 getObject() 方法生成的对象，这个对象的创建是实现类自定义的。</p>
<div class="details admonition tip open">
        <div class="details-summary admonition-title">
            <i class="icon fas fa-lightbulb fa-fw"></i>技巧<i class="details-icon fas fa-angle-right fa-fw"></i>
        </div>
        <div class="details-content">
            <div class="admonition-content"><p>如果一个对象的创建十分复杂，需要设置很多参数、初始化等操作。那么我们可以创建一个FactoryBean实现，专门负责构建这个对象，构建的行为就定义在 getObject() 中。</p>
<p>我们把这个自定义的 FactoryBean 加入到 ioc 容器（在类上添加 @Component 注解等方式），当调用 getBean() 实例化时，最终返回的并非这个 FactoryBean 实现类对象，而是 getObject() 中构建的对象。</p>
</div>
        </div>
    </div>
<h3 id="42-缓存中获取">4.2 缓存中获取</h3>
<p>从上面的代码分析中已经知道，最终调用了 getObject() 之后，会将这个实例缓存到一个变量中，这个变量是 <code>factoryBeanObjectCache</code>。</p>
<p>回到 doGetBean() 中开始的代码块，先从缓存中获取实例，如果获取到了，同样会调用 getObjectForBeanInstance()，判断实例是否是 FactoryBean 类型。</p>
<p>源码如下：
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre tabindex="0" class="chroma"><code><span class="lnt">1
</span><span class="lnt">2
</span><span class="lnt">3
</span><span class="lnt">4
</span><span class="lnt">5
</span><span class="lnt">6
</span><span class="lnt">7
</span></code></pre></td>
<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl">    <span class="n">Object</span> <span class="n">sharedInstance</span> <span class="o">=</span> <span class="n">getSingleton</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">    <span class="k">if</span> <span class="o">(</span><span class="n">sharedInstance</span> <span class="o">!=</span> <span class="kc">null</span> <span class="o">&amp;&amp;</span> <span class="n">args</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// ...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="c1">// 如果bean不是FactoryBean类型或者beanName以&amp;开头，则直接返回。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="c1">// 否则将bean强转为FactoryBean类型，并调用getObject()方法返回。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="n">bean</span> <span class="o">=</span> <span class="n">getObjectForBeanInstance</span><span class="o">(</span><span class="n">sharedInstance</span><span class="o">,</span> <span class="n">name</span><span class="o">,</span> <span class="n">beanName</span><span class="o">,</span> <span class="kc">null</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">    <span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
在 [<a href="/posts/spring/06-cyclic-dependence/#get-singleton-source" rel="">06 循环依赖的实现原理 - getSingleton()</a>] 中有提到，这里可能返回一个bean实例。实际上在 bean 第一次创建后，后面再获取都是从缓存中拿的。拿到了之后，经过一些处理，就返回。这个处理就是判断 FactoryBean 的处理。</p>
<p>从上面源码中可以看到，如果从缓存中获取到，会调用 <code>getObjectForBeanInstance()</code> 。很明显，在第一次创建 bean 实例时，就调用过这个方法，会把创建的结果缓存到 factoryBeanObjectCache 变量中，这里就直接从 factoryBeanObjectCache 变量中获取的实例。</p>
<p>最终，如果这个 bean 是 FactoryBean 类型，会被替换掉。</p>
<h3 id="43-获取原始bean">4.3 获取原始bean</h3>
<p>关于获取原始bean的方式，在 beanName 前加一个前缀 <code>&amp;</code>，就可以获取到原始 bean 实例。</p>
<p>在 getObjectForBeanInstance() 中有这么一段源码：</p>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre tabindex="0" class="chroma"><code><span class="lnt">1
</span><span class="lnt">2
</span><span class="lnt">3
</span><span class="lnt">4
</span><span class="lnt">5
</span><span class="lnt">6
</span><span class="lnt">7
</span><span class="lnt">8
</span></code></pre></td>
<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractBeanFactory</span> <span class="kd">extends</span> <span class="n">FactoryBeanRegistrySupport</span> <span class="kd">implements</span> <span class="n">ConfigurableBeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="n">Object</span> <span class="nf">getObjectForBeanInstance</span><span class="o">(</span><span class="n">Object</span> <span class="n">beanInstance</span><span class="o">,</span> <span class="n">String</span> <span class="n">name</span><span class="o">,</span> <span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="nd">@Nullable</span> <span class="n">RootBeanDefinition</span> <span class="n">mbd</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// 如果bean不是FactoryBean类型或者beanName以&amp;开头，则直接返回。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="k">if</span> <span class="o">(!(</span><span class="n">beanInstance</span> <span class="k">instanceof</span> <span class="n">FactoryBean</span><span class="o">)</span> <span class="o">||</span> <span class="n">BeanFactoryUtils</span><span class="o">.</span><span class="na">isFactoryDereference</span><span class="o">(</span><span class="n">name</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="k">return</span> <span class="n">beanInstance</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">    <span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>看一下 isFactoryDereference() 的源码：</p>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre tabindex="0" class="chroma"><code><span class="lnt">1
</span><span class="lnt">2
</span><span class="lnt">3
</span><span class="lnt">4
</span><span class="lnt">5
</span></code></pre></td>
<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">BeanFactoryUtils</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">public</span> <span class="kd">static</span> <span class="kt">boolean</span> <span class="nf">isFactoryDereference</span><span class="o">(</span><span class="nd">@Nullable</span> <span class="n">String</span> <span class="n">name</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="o">(</span><span class="n">name</span> <span class="o">!=</span> <span class="kc">null</span> <span class="o">&amp;&amp;</span> <span class="n">name</span><span class="o">.</span><span class="na">startsWith</span><span class="o">(</span><span class="n">BeanFactory</span><span class="o">.</span><span class="na">FACTORY_BEAN_PREFIX</span><span class="o">));</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>可以看到，如果 beanName以 BeanFactory.FACTORY_BEAN_PREFIX 开头，就会返回true。</p>
<p>BeanFactory.FACTORY_BEAN_PREFIX 常量的声明：</p>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre tabindex="0" class="chroma"><code><span class="lnt">1
</span><span class="lnt">2
</span><span class="lnt">3
</span></code></pre></td>
<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">interface</span> <span class="nc">BeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">	<span class="n">String</span> <span class="n">FACTORY_BEAN_PREFIX</span> <span class="o">=</span> <span class="s">&#34;&amp;&#34;</span><span class="o">;</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>结合上面的源码，如果 getBean() 时，传入的 beanName 是以 &amp; 开头，走到 getObjectForBeanInstance() 时，就不会执行 FactoryBean 的逻辑，而是直接返回 bean 实例。</p></div>

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